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It started in March, when the American Association for the Advancement of Science released a report on climate change titled What We Know to kick off an initiative to raise awareness on the issue. It makes clear that not only is human-caused climate change real and happening, but that we need to take quick and direct action to rein in greenhouse gas emissions to avert likely catastrophe. Now the the United Nation's Intergovernmental Panel on Climate Change has released a grim report acknowledging that global carbon emissions have continued accelerate even with the recent spike in political rhetoric on tackling the issue.

Despite the growing alarm in the scientific community on climate change, deniers have continued to raise their voices to drown out concern. Two arguments most often used by climate skeptics are that climate change is part of the planet's natural cycle and climate variability and that climate projections rely on fallible computer models. A study released this month in the peer-review journal Climate Dynamics, though, should put these arguments to rest.

Conducted by geophysicist Dr. Shaun Lovejoy of McGill University in Montreal, the study analyzed temperature data collected since 1500, paying particular attention to changes in the past 125 years, since the onset of the Industrial Revolution. Lovejoy considered these temperature changes in the context of longer-term climate fluctuations and looked at records of tree rings, ice cores, cores of the ocean floor and lake sediments. This kind of data offers insight into hemispheric and global climate fluctuations over hundreds, thousands or in some cases even hundreds of thousands of years. For instance, some ice cores from the South Pole can offer a blueprint of climate fluctuations over the past 800,000 years.

After a review of these geological climate records, Lovejoy applied a "fluctuation-analysis technique" -- a method for determining the probability of a given event -- to understand temperature variations over wide ranges of time. He concluded that a global warming event such as the one we have been experiencing over the past century has an incredibly small likelihood, at least one in 1,000. If a bell curve analysis is applied to the data, that likelihood would become even more minuscule, ranging from one in 100,000 to one in 10 million. Lovejoy's study indicates with a confidence greater than 99% that the rate of climate change that has taken place over the past 125 years cannot be ascribed to natural cycles.

"[Climate skeptics] often try to make it appear as though there is a debate on this issue. That's what this study is going to change," Lovejoy says. "I think it's game over for the climate deniers. We can't mess around for another 10 or 20 years."

Lovejoy's research, which was unfunded, uses carbon dioxide as a surrogate for other human-caused and natural events associated with climate fluctuations, including land-use changes and aerosol pollution from volcanic eruptions -- variables that can often stump computer models.

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